The present invention relates generally to a mud flap mounted behind the rear wheels of a vehicle to prevent mud and the like from being thrown to the rear of the vehicle and, more particularly, to a spring mounted mud flap support arm which is deflectable about a support to prevent damage to the mud flap support arm if the mud flap or support arm comes into contact with an obstruction. It is well-known in the prior art to provide deflectably mounted, spring loaded support arms for mounting a mud-flap on a truck or other vehicle. While such structures have in general been quite useful for logging trucks, in highway situations there have been some disadvantages associated with the typical prior art structures. Such prior art structures, as exemplified by U.S. Pat. Nos. 2,652,266 and 2,801,867, have provided for up-and-down movement as well as movement about the vertical axis. When mud or the like builds up on the mud flap, however, the prior art support arms tend to droop due to their inability to to support additional weight. Therefore the support arms do not maintain a constant vertical relationship with respect to the wheel. Also, there is a tendency in such prior art structures for the flaps to bounce and shake when the vehicle to which they are attached goes over small bumps in the road, which bouncing and shaking under some circumstances can have an effect on the life of the flaps. Additional prior art provided by U.S. Pat. No. 4,352,502 provides for a mounting suitable for highway use which allows movement about the vertical axis but does not allow the arm to yield upward should it be struck by an obstruction from below. Prior art provided by U.S. Pat. No. 3,401,953 and Canadian Patent 846-075 discloses a mud flap support arm rotatable in any direction from and normally centered position which and is of sufficient strength to support a mud encrusted mud flap without drooping whether on a smooth or rough road. There is a possibility of failure, however, due to the return spring being anchored to the thin shell of the arm by a nut and bolt. This arrangement is sufficient for most loads but not for an extraordinarily large load such as may be encountered when the mounting arm is crushed between the vehicle tires and an obstacle.